The Thousand Islands constitute an archipelago of 1,864 islands that straddles the Canada-U.S. border in the Saint Lawrence River as it emerges from the northeast corner of Lake Ontario. They stretch for about 50 miles (80 km) downstream from Kingston, Ontario. The Canadian islands are in the province of Ontario, the U.S. islands in the state of New York.

the 1,864 islands range in size from over 40 square miles (100 km²) to smaller islands occupied by a single residence, or uninhabited outcroppings of rocks that are only home to migratory waterfowl. To count as one of the Thousand Islands, emergent land within the river channel must have at least one square foot (0.093 m²) of land above water level year-round, and support at least two living trees.